SVC routing in network with static routing tables

ABSTRACT

A method is disclosed for establishing a switched virtual circuit in a digital network having network nodes with static routing tables. The static routing tables contain at least primary and alternate routing data. When a node is unable to forward a call over its outgoing primary route due to congestion or physical failure and its alternate route is the same as the route on which a call setup request arrived, it clears the call at that node and sends a crankback message to the preceding node, which responds to the crankback message to attempt to dynamically re-route the call over the alternate route stored in the routing table of the preceding node. If the attempt is unsuccessful, it sends the message back to the next preceding node and so on.

This invention relates to a method of establishing a switched virtualcircuit in digital networks with static routing tables. The invention isapplicable to any technology in which switched virtual circuits are setup through a packet switched network, for example, ATM and Frame Relaynetworks.

Congestion may occur on a network link if many incoming streams oftraffic all terminate on the same outbound link, or the outbound linkmay busy or down due to a failure. Congestion is a problem in allnetworks. In packet switched networks, congestion is handled by end toend applications. There are no methods for handling congestion at thepacket routing level. In circuit switched networks, bandwidth isreserved for each circuit by the network operator. Once again, there areno methods for handling congestion.

In networks which use switched virtual circuits (SVCs), a method isneeded to reroute around congested or failed links. In the P-NNIprotocol, which applies to dynamically routed SVC networks, a crankbackIE (Information Element) is used for rerouting. When a node receives acall set-up request with which it is unable to comply, it sends acrankback IE back through the network. This IE passes through severalnodes until it reaches a node which is programmed to respond to acrankback IE. This node, which is equipped to respond to a crankbackelement, then attempts to re-route the call on the basis of its currentrouting table.

This method, however, cannot been used in statically routed SVCnetworks, where the nodes contain manually pre-configured routingtables, which predetermine the path through the network that a callbetween any two endpoints will take. Furthermore, there is no mechanismto prevent continuous crankback attempts. In a network with staticrouting tables, failure in one of the configured links will result infailure of the call set-up process.

Routing loops may occur due to configuration errors, the use ofalternate routes when failures occur, or transient routing tables aftera failure in the network. Routing loops are also a problem in allnetworks. In statically routed packet and circuit switched networks,loops are assumed to be detected by the network operator. No loopdetection mechanisms are present in these networks.

In networks which use switched virtual circuits, both transient loopsand permanent loops are a problem. A reliable method of loop detectionis therefore needed in these networks. The P-NNI protocol uses sourcerouting to avoid loops. This adds unnecessary complexity to SVC routing.

The article “An Alternate Path Routing Scheme Supporting QOS and FastConnection Setup in ATM Networks”. Proceedings of the Global Conference,San Fransisco, Nov. 28-Dec. 2, 1994, vol. 2 of 3, by E. M. Spiegel et.al discloses to an alternate path routing scheme based on a combinationof progressive control and source routing for use within aconnection-oriented data communications network, namely an ATM network.Spiegel teaches that a source node, which is to setup a connectionthrough the network to an endpoint, computes a primary (first) path fromits routing table information. Source routing is utilized to effect thispath computation which typically is a function of link cost and qualityof service requirements. The source node also computes a cost thresholdand crankback limit which, together with the computed primary path, areincluded in a connection setup message generated at the source node.Additionally, the setup message includes fields for accumulating thecost of a path fragment already established and for accumulating thenumber of crankbacks already completed.

The setup message is propagated along the primary path until either thenetwork endpoint is reached or until the connection setup messageencounters a blocked link at an intermediate node. The intermediatenode, responsive to encountering the blocked link, then computes analternate path as a new path tail through the network to the endpoint.Again this path computation is effected using source routing, but inthis instance the node must take into consideration the re-routeguidelines contained in the connection setup message. The total cost ofthe path, comprising the accumulated cost of the established pathfragment together with the cost of the new path tail, must be less thanor equal to the cost threshold specified in the setup message.

If no path tail that satisfies the cost threshold can be found by thenode, and the connection setup message has not yet been cranked back thelimit specified therein, then crankback proceeds to the previous node inthe path fragment and that node in turn attempts to compute an alternatepath using source routing and complying with the constraints specifiedin the setup message.

According to the present invention there is provided a method ofestablishing a switched virtual circuit call in a digital communicationsnetwork comprising a plurality of network nodes, each network nodehaving a local static routing tables providing next hop routinginformation to adjacent nodes, characterized in that said routing tablesdefine a primary route and an alternate route to adjacent nodes; anetwork node receives a setup message for the call and searches itsrouting table for corresponding routing information; the node, based onthe corresponding routing information, attempts to forward the setupmessage on the primary route; if the primary route is not usable due tocongestion or physical failure, the node then attempts to forward thesetup message on the alternate route; and if the alternate route is thesame route on which the setup message is received, the node cranks thecall back to a preceding node which either forwards the setup messageover the alternate route defined in that node's routing table or againcranks the call back to a further preceding node.

Preferably, the crankback IE has a predetermined lifetime to preventcontinuous crankback attempts.

The invention will now be described in more detail, by way of exampleonly, with reference to the accompanying drawings, in which:

FIG. 1 shows a portion of a digital network showing how the systemroutes SVCs around failures; and

FIG. 2 illustrates the network with a different trunk group down andshowing an extra node.

FIG. 1 shows a digital network comprising three switching nodes A, B, C,referred to as node A, node B, and node C, for example, ATM switches,and four users 5, 6, 7, 8, referred to as user 1, user 2, user 3, anduser 4. User 1 and user 4 are connected to Node A, user 2 is connectedto node C, and user 3 is connected to through a “foreign” network 9 toNode B. The nodes are connected via trunks 1 carrying SVCs (SwitchedVirtual Circuits).

Each node contains a local routing table 11 which is manuallypreconfigured when the network is setup to store the routes to adjacentnodes for reaching possible endpoints from that node. This is known ashop-by-hop routing. The routing tables 11 contain information pertainingto a primary route and an alternate route to use in the event that thereis a failure or congestion on the primary route.

Suppose that the Primary Route for Node A, as stored in its routingtable 11, to reach User 2 is via Node B and the Alternate Route is viaNode C. On Node B, the Primary Route to User 2 is via Node C and anAlternate Route is via Node A.

In this example, all Trunk Groups between Node B and Node C are assumedto be down. A Setup message from User 1 to User 2 is routed by Node A toNode B in accordance with the information stored in its routing table11. Node B detects that the Trunk Group in its Primary Route (via NodeC) is down, so its Primary Route cannot be used. Node B also detectsthat its Alternate Route to User 2 is the same Route on which the setupmessage was received. Routing the call out the Alternate Route wouldtherefore cause a loop and Node B therefore determines that it cannotforward the call to User 2 and clears the call back to Node A with aRelease message indicating Crankback.

Node A receives the Crankback message, notes that its Primary Routedidn't work and forwards the call on its Alternate Route, as stored inits routing table 11, to Node C. Node C then forwards the call to User2. Without Crankback, the Trunk Group failure between Node B and Node Cwould prevent User 1 from establishing an SVC to User 2.

If this attempt fails, node A attempts to repeat the process back to thenext preceding node and so on back through the network. In this example,the next preceding node is the originating user 1, so the call iscleared.

Crankback thus operates by regressing a call in progress back to aprevious node in the call setup path to try an alternate route from thatpoint. When a node cannot forward a call along its configured path, itclears the call at that node and sends a Release message to the previousnode (called the Crankback destination) and includes a CrankbackInformation Element in the Release message to indicate Crankback (asopposed to normal call clearing). The Crankback destination has a recordof the Route and Route List it used to forward this call. Upon receivingthe Crankback, the Crankback destination knows that the Route it chosecannot be used to establish this call. It therefore chooses the nextbest Route from the Route List to forward the call. This newly-selectedRoute will take the call to a different node in an attempt to routearound the failure. If there are no other Routes to chose from in theRoute List, then the call is cranked back one more hop. If Crankbackreaches the originating node and an alternate Route cannot be found,then the call is cleared back to the calling party.

In order to bound the number of SVC re-routing attempts, Crankback willbe attempted only up to a maximum number of times before the call iscleared back to the originator. This is controlled using a hop count ortime-to-live field in the signalling message. This field starts at aspecific value and is decremented each time the signalling message isforwarded by an intermediate switch. When the field reaches apredetermined value, for example, zero, the call is cleared.

The use of static routing with alternate routes may also cause routingloops during failure conditions. Manually configuring SVC routing tablesnode by node, can also cause routing loops to be erroneously introduced.Loop detection is used to detect a looped Set-up or Add Party messageand will either clear the call or use Crankback to indicate to previousnodes in the setup path that an alternate route must be chosen.

In the network shown in FIG. 2, and additional node 10, referred to asnode D is located between node C and user 2. If we assume that user 1 isestablishing an SVC to User 2, node A's Primary Route to User 2 is viaNode C, with its Alternate Route via Node B. The Primary Route from NodeC to User 2 is via Node D with an Alternate Route via Node B.

When the Setup message arrives at Node C from Node A, Node C determinesthat its Primary Route is inoperative, and forwards the call along itsAlternate Route, to Node B. Node B sends the call along its primaryroute to Node A. Node A detects the routing loop and clears the callback to Node B in a Release message indicating Crankback. Node B thenchooses its Alternate Route and forwards the call to Node D, whichdelivers the call to User 2.

Looping of SVC Setup or Add Party messages can occur due to routingconfiguration errors or due to the use of alternate routes when failuresoccur. SVC looping in case 2 is detected with the use of a proprietaryTransit List Information Element (IE) in the signalling messages. ThisIE (Information Element) records the nodes visited during the callsetup. Upon detection of SVC looping, the detecting node cranks the callback to the previous node to allow rerouting to occur.

The described method enhances the crankback mechanism and allows its usein statically routed SVC networks. It also prevents looping in a simpleand effective manner.

What is claimed is:
 1. A method of establishing a switched virtualcircuit call in a digital communications network comprising a pluralityof network nodes, each network node having a local static routing tableproviding next hop routing information to adjacent nodes, comprising thesteps of: defining in said routing table a primary route and analternate route to adjacent nodes; sending a setup message for said callthrough said network; receiving said setup message at a network node,said network node on receipt of said setup message searching its routingtable for corresponding routing information; and said network node,based on the corresponding routing information, attempting to forwardthe setup message on the primary route; and if the primary route is notusable due to congestion or physical failure, said network node thenattempting to forward the setup message on the alternate route; and ifthe alternate route is the same route on which the setup message isreceived, the network node then cranking the call back to a precedingnode which either forwards the setup message over the alternate routedefined in said preceding node's routing table or again cranks the callback to a further preceding node, and such process being repeated untilthe call setup message has been returned to a source thereof.
 2. Amethod as claimed in claim 1, wherein the step of cranking the call backto said preceding node includes sending a release message having acrankback indication from the network node to the preceding node; and,responsive to the crankback indication, the preceding node eitherforwards the setup message over an alternate route, if it can do so, oragain cranks the call back to another preceding node until the callsetup message has been returned to the source.
 3. A method as claimed inclaim 2, wherein the call setup message and crankback indication have apredetermined lifetime to prevent continuous attempts of cranking backthe call.
 4. A method as claimed in claim 3, wherein the predeterminedlifetime is a count of nodes that can still be visited during the callsetup, said count is decremented each time the setup message isforwarded by a network node, and the call is cleared when the countreaches zero.
 5. A method as claimed in claim 2, wherein said call iscleared when the release message with the crankback indication reachesthe source.
 6. A method as claimed in claim 1, wherein the primary oralternate routes are not usable due to congestion or physical failure,or looping is detected.
 7. A method as claimed in claim 6, whereinlooping is detected when the primary route or the secondary route is thesame as the route on which the setup message is received by the node.